Rhombomere interactions control the segmental differentiation of hindbrainneurons

The embryonic hindbrain is subdivided into a series of metameric units term ed rhombomeres, which display features that strongly suggest they are auton omous developmental units. However, some aspects of their phenotype develop nonautonomously. Here we have analyzed the possibility that interrhombomer e interactions generate the pattern of segmental neuronal differentiation. The differentiation of both projection interneurons and motor neurons in th e hindbrain is retarded in rhombomeres 3 and 5. We demonstrate here that if either rhombomere 3 or 5 is isolated from the influence of their neighbour s, either in vitro or in vivo, then these segments no longer display delaye d neuronal diufferentiation. We further show that the retardation of motor neurons differentiation in rhombomeres 3 and 5 is, at least in part, mediat ed by Bmp-4. If this molecule is inhibited, by grafting cells expressing ch ordin, then the motor neurons of these rhombomeres develop ahead of their n ormal schedule.